The invention is directed to light emitting diodes and in particular to ohmic contacts for light emitting diodes.
One technique for measuring the efficiency of light emitting diodes (LEDs) is by determining the luminance per watt. The luminance provided by light emitting diodes is dependent upon several factors such as internal quantum efficiencies, as in the case of an injected carrier not being converted to a photon, or extraction efficiency, as in the case of a small fraction of photons being successfully extracted from the light emitting diode as opposed to being lost to internal absorption. To realize high efficiency LEDS, both of these issues need to be addressed. The potential gain from improving extraction efficiency, however, is likely to be greater and simpler to accomplish than the gains from improving internal efficiency.
One technique to improve light extraction of visible light nitride LEDs, such as Gallium Nitride (GaN) LEDs, is achieved through use of high reflectivity metallurgies which are typically mounted to one side of the LED. GaN based devices typically require ohmic contact formation as a means of establishing electrical contact to the device with minimal impact on the operating voltage of the device. Thus, the high reflectivity metallurgies are typically employed in the ohmic contact and attached to a p-type GaN layer of the LED. One common approach is to use a silver containing layer in the ohmic contact. Silver is desirable, because of its high reflectance. The difference in the work function between silver and the other materials from which the LED is fabricated has been problematic. For example, it is widely accepted that metals with high work functions form the best contacts for p-type semiconductor materials, while metals with low work functions form the best contacts for n-type semiconductor materials. However, surface contamination of the metal semiconductor interface may degrade the ohmic contact performance of metals. Contamination layers at the interface may produce an unforeseen electronic state that may degrade the efficiency of the LED.
There is a need, therefore, to provide improve ohmic contact techniques for LEDs.